Operators of decentralized energy generation plants which feed electrical energy into a public electricity grid at a grid connection point generally have to adhere to provisions set by the grid operator during feeding, namely at the grid connection point. Thus, it is in particular not sufficient to fulfill these provisions, for example in respect of the reactive power to be fed in order to stabilize the grid voltage, at the output of one or more inverters of the respective energy generation plant because this is in no way equivalent to the provisions also being met at the grid connection point. Reasons for this are, for example, mutual influencing of the individual inverters and in particular the effects of connection equipment, which connects the individual inverters to the grid connection point and therefore also to one another. The connection equipment can have very different properties for different inverters of an energy generation plant, for example when the connection equipment connects inverters which are distributed far apart from one another, as usual in photovoltaic systems, to one another and to the grid connection point.
The mutual influencing of inverters can gain increased importance in particular when the inverters and/or the current sources connected thereto vary to a greater extent. In this case, a current source in the form of a generator, on the one hand, and a current source in the form of a storage system for electrical energy, on the other hand, in which the connected inverter can not only output current but can also draw current, is only one example of such a variance.
It is known from “Erläuterungen zu den Vorgaben der EnBW Regional AG zur Blindleistungs-Spannungskennlinie Q(U) für Erzeugungsanlagen am Mittelspannungsnetz (Stand 08/2011)” [Explanations in respect of provisions set by EnBW Regional AG for the reactive power/voltage characteristic Q(U) for generation plants on the medium-voltage grid (version 08/2011)] that when a Q(U) characteristic is required which needs to be realized at the grid connection point, the connection equipment between the inverters and the grid connection point needs to be taken into consideration. For this purpose, two concepts are described. The first is based on measurements at the grid connection point and the inclusion of all of the inverters in a control loop. The other is based on a projection of the inverters onto the grid connection point. For this purpose, an equivalent circuit diagram of the respective connection equipment is stored in the individual inverters and is used for such modification of the control curves of the inverters that the effect of the respective connection equipment up to the grid connection point is compensated for. The inclusion of all of the inverters in a control loop requires quick communication links to the grid connection point and all of the inverters when presets are intended to be precisely adhered to at the grid connection point. The projection of the individual inverters onto the grid connection point requires detailed information on all components parts of the connection equipment and the interconnection of said component parts, for this purpose.
DE 10 2010 006 142 A1 discloses an energy portal for controlling or regulating an energy feed from a generation unit grid into an energy distribution grid. The energy portal comprises an operational parameter control apparatus for controlling or regulating operational parameters of the generation unit grid on the basis of evaluated measured variables, feed factors and prognosis information with the feed factors as a controlled variable. The operational parameter control apparatus can be designed for dynamic control or regulation of reactive power compensation on the basis of measured variables with respect to reactive power of the generation unit grid.
DE 10 2009 030 725 A1 discloses a wind farm comprising a multiplicity of wind turbines, whose generated electrical energy is transferred to an electricity grid at a connection point. The public electricity grid presets setpoint values for the connection point, and a sensor measures actual electrical values at the connection point. A master controller determines a preset for a second control layer on the basis of a difference between upper setpoint values and upper actual values on a first control layer. A plurality of submaster controllers on the second control layer takes the presets as lower setpoint values and makes presets for the wind turbines on the basis of a difference between the lower setpoint value and a lower actual value. Over all control layers there is a closed control loop between the connection point and the wind turbines, with the fault-free operation of said closed loop being dependent on the fact that the correct actual values are supplied to the control modules. The actual values can be measured directly or calculated at least partially from measured values at other points, for example the actual value relevant for one of the submaster controllers from the actual values of the wind turbines assigned thereto. For the stability of such control which is nested in the form of a cascade, it is considered advantageous if the lower control layers each have a smaller time constant than the upper control layers. The submaster controller takes into consideration the fact that the wind turbines are arranged at different distances from the connection point in its presets to the wind turbines for the provision of a quantity of reactive power and matches the presets to the wind turbines correspondingly. In the known wind farm, control modules are provided which can optionally be used as master controller or submaster controller.
There still is a need of a method for controlling a plurality of inverters, which are connected to a current source each on their input side and to a common grid connection point on their output side, by which method presettings at the grid connection point can be adhered to precisely even in case of complex connection equipment, but which method can be implemented without excessive complexity being involved.